New immunolabels which contain highly monodisperse, biologically compatible 3 to 20 nm metal particles encapsulated within coordination shells will be synthesized using three approaches: (i) Cluster complexes of gold and platinum will be enlarged by catalytic metal deposition and ligand capping. (ii) Apoferritin will be modified by site-specific mutagenesis to incorporate amino acid residues which can specifically bind gold or platinum on its interior surface; these will then be used to nucleate the formation of 6 nm metal particles inside the assembled protein. (iii) Cage-like molecules will be constructed around known gold cluster complexes, using rigid rod supports and cross-linked flat plates to construct a spherical shell, which will control the final size of a colloidal particle generated by metal deposition onto the cluster. New labels will be covalently conjugated to antibodies and proteins to give novel probes which combine the ease of visualization and size choice of colloidal gold with the high specimen penetration, freedom from aggregation and high sensitivities of heavy metal cluster immunoprobes. New probes will be evaluated in collaborations to study DNA replication in the cell nucleus, and detection and characterization of Microsporidia responsible for opportunistic infections associated with the Human Immunodeficiency Virus. PROPOSED COMMERCIAL APPLICATION: New immunoprobes will be developed which combine the size choice (from 3 to 20 nm) and ease of visualization of colloidal gold with the high penetration, sensitivity and low non- specific binding of heavy metal cluster probes. These could potentially replace conventional colloidal gold probes in most of their applications. The proposed synthetic approaches may also contribute to the development of novel biomedical imaging agents, therapeutics, biosensors and new materials.